water piston. The steam piston of one side operates mechanically the valve of the opposite side. A certain lost motion, or valve rod clearance, M, is provided in the transmitting mechanism. This retards the motion of the valve and allows the steam piston to finish its stroke. The steam cylinder is equipped with double ports, the outer one being the steam port and the inner one the exhaust port. The piston in approaching the head closes its own exhaust port, entraps the steam and by compressing it is stopped by this steam cushion. Larger pumps are provided with valves for regulating the cushion.
Facias Steam End R.H. L.H.
No. 2 No.l
FIG. 2.—Simple cylinder direct-acting duplex pump.
Duplex pumps give an almost uniform flow of water and require only small air chambers. The speed is limited and those given in Table I should not be exceeded. The economy is low, because no cut-off is possible so that the steam is exhausted at full pressure and its expansive energy is not utilized.
Compound duplex pumps are direct-acting pumps with a high- and a low-pressure steam cylinder arranged tandem on each side and connected directly to the water end. The high-pressure steam cylinder takes steam full stroke, and the steam then exhausts through the intermediate space into the low-pressure cylinder. In this way part of the expansive force of the steam is utilized. Figure 3 shows a longitudinal section through a compound steam end and an end view with a cross-section in the high-pressure cylinder. Packing between the cylinders is here provided by a close-fitting floating sleeve. Stuffing boxes may be used for the same purpose. The intermediate spaces, consisting of the high-pressure exhaust pipes, the low-pressure steam chests and the ports, are connected by the cross exhaust pipe 6, with a valve c inserted for adjustment.